PSI - Issue 53

Serhii Lavrys et al. / Procedia Structural Integrity 53 (2024) 246–253 Serhii Lavrys et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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phase. Predominant dissolution is mainly concent rated on equiaxed α -grains, which indicates the realization of microgalvanic pairs (Fig. 5). It can also be concluded that the lower the corrosion rate, the less relief (rough) the corroded surface.

Fig. 5. Corrosion rate and SEM image of corroded surface of Ti6Al4V titanium alloy after 522 hours immersion in 20 wt.% HCl : wrought ( 1 ), AM ( 2 ), AM after post HT at 800°C ( 3 ) and 850°C ( 4 )

Taking into account the fact that during corrosion tests in chlorine-containing environments, the wrought titanium alloys are mainly exposed to crevice corrosion [5, 6, 13-16], it can be assumed that the corrosion processes for AM Ti6Al4V titanium alloy will also proceed according to this mechanism . That is, during corrosion tests, the passive film of titanium alloys is exposed to the aggressive action of chlorine anions, which leads to the formation of cracks in the passive film, the release of titanium ions and their subsequent interaction with chlorine. That is, when the passivity is locally broken, and the oxygen that can repassivate the system is exhausted, titanium dissolution occurs inside the crack (Fig. 6).

Fig. 6. Scheme of crevice corrosion mechanism of wrought and AM Ti6Al4V titanium alloy in 20 wt.% HCl

However, despite the same corrosion mechanism, AM titanium alloys have worse corrosion resistance than wrought ones. It is obviously that this deterioration of corrosion resistance is related to the structural features of AM titanium alloy. For example, Wu B. et al. noted that the metas table martensitic α' phase has « higher energy state » , which leads to higher intensity of its dissolution during corrosion tests compared to stable α or β phases [10]. Residual stresses present in AM titanium alloy will also negatively effect the dissolution resistance due to the acceleration of diffusion processes. Therefore, carrying out post HT allows to improve the corrosion resistance of AM titanium alloy due to the reduction of both the metastable α' phase and residual stres ses in the material. It should also be noted that during HT, the β phase is released, which is more stable than the α or αˊ phases and, as a result, improves the corrosion resistance [11 – 14]. However, HT at higher temperature (850 ° C), although it provides lower values of residual stresses and larger amount of β phase, leads to lower corrosion resistance of AM titanium alloy compared to HT at lower temperature (800 ° C). This can be explained by the fact that besides of the above mentioned structural features that effect the corrosion resistance, the grain sizes should also be taken into account. Since the higher temperature of post HT leads to a coarsening of the structure and an increase of the size of α and β grains, which negatively effects the resistance to corrosion dissolution of AM titanium alloy [13, 14]. In summary, post HT under appropriate time-temperature regimes is an effective method of improving the anti-corrosion characteristics of titanium alloys. However, it is necessary to avoid high temperatures of post HT, as a potential factor of coarsening of structural components, which will negatively effect the corrosion resistance of AM